The invention relates to a bulb designed for emitting radiation through an emission surface of an envelope which contains a radiation source.
In most known bulbs, the envelope has the shape of a cylindrical body of revolution, while the radiation source is formed by a filament or a cylinder of small diameter arranged on the axis of revolution of the envelope. Such a bulb is known from French patent no. 1,270,856. The radiation emitted by a bulb of this type manifests itself in the form of cylindrical heat waves whose axis is that of the envelope. When such a bulb is used for heating an object which has a planar receiving surface, such as the bottom of a vessel or a sheet of paper, the heat distribution as received by said surface is inhomogeneous, the regions of the receiving surface closest to the axis of the envelope being submitted to a more intense heat than the regions of the receiving surface which are farther removed from said axis. Such inhomogeneity has adverse effects, because a nominal heating level of the regions of the receiving surface farthest removed from the axis of the envelope may cause an overheating of the regions closest to this axis, and may thus damage the item to be heated, while on the other hand a nominal heating of the regions of the receiving surface closest to the axis of the envelope will imply an insufficient heating of the regions farthest removed from the axis.
Furthermore, the surface power density of the radiation emitted by the known bulbs is comparatively low, which translates itself into a low power efficiency.
The invention has for its object to remedy the above disadvantages by proposing a bulb capable of emitting a homogenous radiation towards a planar surface with a high radiation density.
To achieve this in a bulb according to the invention, the emission surface is substantially plane, and the radiation source defines a planar surface substantially parallel to the emission surface.
The planar shape of the emission surface and of the radiation source enables the bulb according to the invention to generate a radiation which arises in the form of planar heat waves, allowing a homogeneous heating of a planar receiving surface, provided the latter is arranged parallel to the emission surface. Moreover, the surface power density of the radiation emitted by the bulb according to the invention, and accordingly the efficiency of the heating operations carried out by means of said bulb, will be a function directly of the ratio between the surface defined by the radiation source and the emission surface, and may be adjusted through the design of the radiation source.
In a special embodiment of the invention, the radiation source is formed by at least one filament of flat shape.
In another special embodiment of the invention, the radiation source is formed by a plurality of co-planar filaments.
In another special embodiment of the invention, the radiation source is formed by at least one convoluted filament.
In a preferred embodiment of the invention, the radiation source is formed by a reactive gas which is to be excited by means of electrodes.
In a variant of the invention, the envelope has a reflecting surface arranged opposite the emission surface.
The reflecting surface enables to increase the surface power density of the radiation emitted by the bulb, and thus to increase even more the efficiency of the heating operations carried out by means of said bulb.
In another variant of the invention, the envelope has a surface which is convex towards the exterior of the envelope situated opposite the emission surface.
Such a convex shape of the surface situated opposite the emission surface will make it easier to position the bulb in a cavity provided within a lamp designed to accommodate the bulb. Moreover, if the convex surface is covered with a reflecting layer, part of the radiation emitted by the bulb will be concentrated towards the center thereof, which will render the construction of the radiation source easier in certain embodiments.